remove zero columns in zonotope constructor

src/Zonotope.jl

@@ -42,6 +42,10 @@ ball in ``\\mathbb{R}^n`` by an affine transformation.
             generators_list::AbstractVector{VN}
            ) where {N<:Real, VN<:AbstractVector{N}}`
 
+The optional argument `remove_zero_generators` controls whether we remove zero
+columns from the `generators` matrix.
+This option is active by default.
+
 ### Examples
 
 A two-dimensional zonotope with given center and set of generators:
@@ -87,12 +91,22 @@ julia> Z.generators
 struct Zonotope{N<:Real} <: AbstractCentrallySymmetricPolytope{N}
     center::AbstractVector{N}
     generators::AbstractMatrix{N}
+
+    function Zonotope(center::AbstractVector{N}, generators::AbstractMatrix{N};
+                      remove_zero_generators::Bool=true) where {N<:Real}
+        if remove_zero_generators
+            generators = delete_zero_columns(generators)
+        end
+        new{N}(center, generators)
+    end
 end
 
 # constructor from center and list of generators
-Zonotope(center::AbstractVector{N}, generators_list::AbstractVector{VN}
+Zonotope(center::AbstractVector{N}, generators_list::AbstractVector{VN};
+         remove_zero_generators::Bool=true
         ) where {N<:Real, VN<:AbstractVector{N}} =
-    Zonotope(center, hcat(generators_list...))
+    Zonotope(center, hcat(generators_list...);
+             remove_zero_generators=remove_zero_generators)
 
 
 # --- AbstractCentrallySymmetric interface functions ---

test/unit_Zonotope.jl

@@ -40,6 +40,13 @@ for N in [Float64, Rational{Int}, Float32]
     d = N[0, -1]
     @test σ(d, z) == N[2, 1]
 
+    # zero column in generators
+    g = zeros(N, 2, 5)
+    g[:, 3] = ones(N, 2)
+    g[1, 2] = N(2)
+    z = Zonotope(N[1, 2], g)
+    @test size(z.generators) == (2, 2)
+
     # boundedness
     @test isbounded(z)
 
@@ -100,6 +107,9 @@ for N in [Float64, Rational{Int}, Float32]
     Z = convert(Zonotope, Hyperrectangle(N[2, 3], N[4, 5]))
     @test Z.center == N[2, 3] && diag(Z.generators) == N[4, 5]
     convert(Zonotope, BallInf(N[5, 3], N(2)))
+    # flat hyperrectangle
+    Z = convert(Zonotope, Hyperrectangle(N[2, 3], N[0, 0]))
+    @test Z.center == N[2, 3] && isempty(Z.generators)
 
     # convert the cartesian product of two hyperrectangles to a zonotope
     h1 = Hyperrectangle(N[1/2],  N[1/2])